Re: SSD + RAID

On Fri, Nov 13, 2009 at 12:22 PM, Scott Carey
<scott(at)richrelevance(dot)com> > On 11/13/09 7:29 AM, "Merlin Moncure"
<mmoncure(at)gmail(dot)com> wrote:
>
>> On Fri, Nov 13, 2009 at 9:48 AM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
>> wrote:
>>> I think RAID6 is gonna reduce the throughput due to overhead to
>>> something far less than what a software RAID-10 would achieve.
>>
>> I was wondering about this.  I think raid 5/6 might be a better fit
>> for SSD than traditional drives arrays.  Here's my thinking:
>>
>> *) flash SSD reads are cheaper than writes.  With 6 or more drives,
>> less total data has to be written in Raid 5 than Raid 10.  The main
>> component of raid 5 performance penalty is that for each written
>> block, it has to be read first than written...incurring rotational
>> latency, etc.   SSD does not have this problem.
>>
>
> For random writes, RAID 5 writes as much as RAID 10 (parity + data), and
> more if the raid block size is larger than 8k.  With RAID 6 it writes 50%
> more than RAID 10.

how does raid 5 write more if the block size is > 8k? raid 10 is also
striped, so has the same problem, right? IOW, if the block size is 8k
and you need to write 16k sequentially the raid 5 might write out 24k
(two blocks + parity). raid 10 always writes out 2x your data in
terms of blocks (raid 5 does only in the worst case). For a SINGLE
block, it's always 2x your data for both raid 5 and raid 10, so what i
said above was not quite correct.

raid 6 is not going to outperform raid 10 ever IMO. It's just a
slightly safer raid 5. I was just wondering out loud if raid 5 might
give similar performance to raid 10 on flash based disks since there
is no rotational latency. even if it did, I probably still wouldn't
use it...

merlin